This invention relates to heat exchangers used in thermal management systems for systems having thermal management problems. In particular, the invention relates to a modified tube and shell heat exchanger for the transfer, storage, and dissipation of thermal energy generated in high-power, ground-based or space-based power systems applications, such as nuclear power-related devices, and microwave devices.
In the operation of high energy power systems, thermal energy generated during operation must be conveyed away from the source and dissipated. In systems demanding dissipation of high thermal loads, such as generated during a burst power mode, thermal energy may be conveyed from the source, temporarily stored, and later conveyed to an ultimate heat sink, such as the environment or outer space.
To meet such demands it has been proposed to use a phase-change material, such as lithium salts, as a heat sink to remove and temporarily store excess heat generated during burst power modes. Thereafter, the heat may be dissipated from the heat sink over a longer period of time to an ultimate heat sink. Phase-change materials have a high heat of fusion, which enables the storage of significant amounts of thermal energy in such materials as they change phase from solid to liquid phase. They later resolidify as thermal energy is dissipated to an ultimate heat sink. Heat sinks comprising phase-change materials for temporary storage of thermal energy may be located near the heat source or located away from the heat source. The present invention best suited for use in thermal management systems wherein thermal energy storage is located away from the heat source, is also valuable for use in such systems where storage is near the heat source.
While lithium hydride has a high thermal energy storage capacity, it has a very low thermal conductivity. Accordingly, when used in heat sinks, the low thermal conductivity presents an obstacle to the rapid and efficient transfer of heat from the heat source into phase-change materials for storage. In applications where thermal energy is to be stored away from the heat source, systems have been proposed wherein thermal energy is conveyed by a cooling fluid from the heat source to a modified tube and shell heat exchanger for storage. As proposed, the tubes disposed in such modified heat exchangers contain heat sinks in which thermal energy is stored, rather than containing a second cooling fluid into which the energy is exchanged, as in conventional tube and shell heat exchangers. Thus, as adapted for thermal storage systems, the tube and shell design may comprise one or more heat sink tubes which are surrounded by a larger shell. A cooling fluid is circulated in the space defined between the tube and shell. In high power applications the cooling fluid may be sodium. The cooling fluid is typically circulated by one or more pumps through a closed loop circuit of piping from the heat source to the heat exchanger and back.
The short time constraint for removing energy during burst power modes and the poor thermal conductivity of phase change materials have dictated that such modified heat exchangers employ small cylindrical heat sink tubes and high cooling fluid speeds. This puts a particularly serious constraint on using the modified tube and shell heat exchangers for heat storage in space-based systems as larger and more massive systems components such as pumps are required. The need exists, therefore, to find means for enhancing the heat transfer into heat sinks from cooling fluids and provide the rapid and efficient storage of heat into modified tube and shell heat exchangers for space-based, as well as ground-based, applications.